This invention relates to CMG (Control Momentum Gyros), in particular, mitigating gimbal induced disturbances in CMG arrays.
CMGs are commonly used for attitude control in satellites and other spacecraft. A CMG 10 in FIG. 1 on a spacecraft 12 includes an inner gimbal assembly (IGA) 14 and a gimbal torque actuator (GTA) or motor 16 which rotates the IGA. The IGA is an assembly that is free to rotate in one or more axes 17 and stores angular momentum in its rotating inertial mass (rotor) 18, rotated at a constant speed by a rotor spin motor (RSM) 20. Output torque to rotate the spacecraft around on axis is obtained from the CMG by rotating the IGA 14. In practice, several CMGs are aligned along different axis so that the satellite can be oriented spherically.
Output torque on axis 22 from the CMG is the result of the mathematical cross product of the gimbal precession rate vector and the angular momentum vector of the IGA. The output torque is in a plane formed perpendicular to each CMG gimbal axis. Spacecraft attitude control is achieved through the coordinated actuation of a plurality of CMGs in a CMG array. This is a well understood mechanical process that has been employed in spacecraft control for some time. As stated, the rotational torque applied to the satellite from the CMG is a cross product of the IGA rate and stored angular momentum of the rotor 16. An IGA rate can be used to produce high output torque by storing high angular momentum in the IGA rotor. This process is often referred to as torque multiplication because a small input torque to the IGA (input axis in FIG. 1) is multiplied by the stored angular momentum (spin axis) to create a high output torque (output axis).
A problem in the practical application of CMG arrays is noise disturbances in the IGA actuation, which also multiplied/amplified and transmitted to satellite motion, producing less than smooth satellite movement. These disturbances are undesirable in the control of precision pointing spacecraft. The most pervasive gimbal disturbances are those associated with the gimbal rate sensor 24, which is used to feed back IGA motion in a closed loop IGA control that controls the IGA motor speed. Noise in sensors 24 errors can not be easily compensated using conventional control techniques.
Another related device is a reaction wheel assembly (RWA), in which the rotor speed is changed to produce rotational torque on the satellite to change its attitude. But the rotors of the RWA array typically are not mounted on gimbals, so their orientations with respect to the spacecraft coordinates do not change like the gimbals of a CMG.
An object of the invention is to mitigate the effect of disturbances and noise in IGA motion in a CMG array.
According to the invention, to mitigate or reduce the effect of disturbances or noise, such as noise from IGA motion, the IGA rotor is operated like an RWA as a function of the disturbance.
According to the invention, the nature of the IGA disturbance is sensed or understood and processed by a signal processor in the CMG system to slightly change the rotor speed to offset the effect of the disturbance. This operation is performed at the array level in a CMG array (CMGA) in an attitude control to mitigate noise in all the CMGs, i.e. all the rotational axis.
A feature of the invention is that is can be employed easily in current CMG controls.
Other objects, benefits and features will be apparent to one of ordinary skill in the art from the following drawing and description.